1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Description of Related Art
The electric power steering apparatuses include a so-called rack parallel type. In the electric power steering apparatus of this type, an electric motor for steering assist is mounted to a housing of a steering mechanism and an output shaft of the electric motor is disposed in parallel to a rack bar of the steering mechanism. A rotary driving force of the electric motor, as a steering assist force, is transmitted to a ball screw mechanism via three gears sequentially meshed with each other, that is a first gear, a second gear and a third gear in this order. Then, the driving force is transmitted to the rack bar (see, for example, Patent Document 1: Japanese Unexamined Patent Publication No. 10-278813).
According to the above document 1, the housing comprises first, second and third housings. The first gear is supported by the first and second housings. The second gear is supported by the first housing in a position-adjustable manner such as to adjust gear-to-gear backlash. The third gear is supported by the third housing.
The following problem arises if the gear-to-gear backlash is significant. When a vehicle is traveling straight on a rough road so that the individual gears do not transmit the power, for example, noises of gear rattling occur due to the influence of reaction force (inverse input) from the road surface.
According to Document 1, a support shaft supporting an intermediate gear is fitted in an elongate hole of the first housing to adjust backlash. However, a clearance tends to be formed between an outer periphery of the support shaft and an inner periphery of the elongate hole. Because of the influence of the clearance, the support shaft is decreased in support rigidity, involving likelihood of increasing gear alignment errors. Specifically, the center axes of the three gears are prone to incline relative to each other.
The large gear alignment errors lead to the following problem. Specifically, the teeth of gears meshed with each other for transmitting power during a steering operation tend to slide easily on each other. This results in the increase of noises caused by the gear sliding noises.
It may be possible to adopt a so-called matching assembly in order to reduce the gear alignment errors and the backlash between each pair of gears. Specifically, the individual parts of the first, second and third gears and of the first, second and third housings are sorted out according to respective degrees of dimensional accuracy variation so that parts with proper accuracies are assembled to each other.
However, the matching assembly requires much labor and time for sorting out the individual parts, resulting in increased manufacture costs.
The invention seeks to provide an electric power steering apparatus to reduce noises and assemble easily.